Micropuncture, microperfusion and microelectrode studies are proposed to develop a better understanding of the transport processes of strong electrolytes across single nephrons in vivo. The translocation of various ion species across the tubular epithelium involves the transfer across two membrane systems in series, i.e., the luminal and peritubular cell boundary. Our experiments will deal with a detailed characterization of the transport properties of individual cell membranes of proximal and distal tubules and with the mechanisms regulating renal epithelial ion transfer. We propose to carry out studies on mammalian and amphibian tubules concerned with: (1) A detailed characterization of proximal tubular fluid transfer with special attention to the role of sodium, chloride and hydrogen ion transport in the regulation of this transport process. (2) An investigation of the role of the distal tubular epithelium in the response to extracellular volume expansion, with particular attention to peritubular physical factors. (3) Further characterization of the extracellular shunt pathway across proximal tubules by a combination of voltage-clamp and tracer studies. (4) The relationship between Na and K transport across the peritubular membrane of distal tubules. (5) The role of the distal tubule and collecting ducts in the response to various types of K-loading and acid-base disorders. (6) A detailed study of the kinetics of distal tubular potassium transfer. (7) Mechanism of tubular chloride transport and (8) Electrophysiological studies to gain information on the permeability and pump properties of single tubule cells.